1. Field of the Invention
This invention relates to an information recording medium.
2. Description of the Prior Art
With an increase in demands for the recording of a variety of information signals in a high recording density, information signals are in recent years recorded and reproduced in a higher density by the use of information recording mediums prepared on the basis of various principles of constitution or principles of operation. For example, there is an information recording medium in which information signals are recorded on a recording layer of the information recording medium by causing a physical change or chemical change corresponding with the information signals as a result of irradiation with a recording beam whose intensity has been modulated according to the information signals. Such an information recording medium includes an information recording medium belonging to a phase change type in which a material (an inorganic material or an organic material) having two or more stable structural states different in their optical and/or electrical characteristics (e.g., light transmittance, reflectance, absorbance, electrical resistance, and other characteristics) and capable of causing a transformation between the above stable structural states as a result of external application of optical, electrical or thermal energy is formed into a film on a substrate by vacuum deposition or sputtering. In respect of such an information recording medium, researches and development have been energetically made so that it can be put into practical use as an optical disk to which users can add records only once (a writing once recording disk) or an erasable optical disk as used in an office file memory and so forth. Proposals are also made on various types of recording mediums that are roughly grouped into a photomagnetic type, a pit formation type, and a bubble formation type. These are well known in the art. Recently, as an information recording medium capable of recording an information signal, targeted as a subject of recording, in the form of a charge image having a high resolution, a proposal is also made on a recording medium provided with a charge retension layer having the function of optical modulation.
In conventional many information recording mediums, however, the recording layer must be formed by vacuum deposition or sputtering, and hence there is the problem that a large-scale installation is required in the manufacture. In addition, there is another problem that information recording mediums endowed with desired characteristics can not be mass-produced with ease in a good yield. Other problems are also pointed out such that a recording material used for the constitution of a recording layer has toxicity and also the recording layer has a poor storage stability (weathering resistance). In respect of information recording mediums that utilize transformation between crystals of an alloy type, there is a disadvantage that they have a poor energy absorption efficiency and a low recording sensitivity. Furthermore, it is also questioned that information recording mediums in which a recording material of an organic type is used can not give good characteristics.
In the case when information recording mediums are used as OHP (overhead projection) sheets, conventional OHP sheets are well known to comprise a transparent resin sheet to which a toner is adhered by an electrophotographic process so that any desired information can be displayed. Such a toner used in development, however, can not be peeled and hence the display can not be rewritten. Accordingly, the sheet once having been used must be discarded. This has brought about a disadvantage of wastefulness, and it has been sought to provide an information recording medium usable as a rewritable OHP sheet.
As an information recording medium that utilizes a polymer liquid crystal composite, Japanese Laid-open Patent Application No. 63-124024 discloses a liquid-crystal-dispersed light valve, in which, as shown in FIG. 13, a polymer-dispersed liquid crystal memory film 100 comprising a liquid crystal dispersed in a polymer binder is held between a pair of transparent electrode substrates 104 each having a transparent electrode 102, and an electrode 106 is connected between transparent electrodes 102.
In such a light valve, information can be written or read by a thermal means in the polymer-dispersed liquid crystal memory film 100, which is in a transparent state with aligned liquid crystals, e.g., by irradiation with a laser beam or using a thermal head. The information thus recorded can be erased by heating and cooling the polymer-dispersed liquid crystal memory film 100 while applying a voltage between the two transparent electrodes 102 through the electric source 106, so that the polymer-dispersed liquid crystal memory film 100 returns to the original transparent state.
Since, however, in such a conventional information recording medium the polymer-dispersed liquid crystal memory film 100 is held between the transparent electrode substrates 104, a transparent electrode substrates 104 is interposed between an information recording layer, i.e., the polymer-dispersed liquid crystal memory film 100, and a recording means. Hence, the heat from the recording means is diffused when transmitted to the polymer-dispersed liquid crystal memory film 100, resulting in a lowering of resolution of information records. Thus, it has been sought to provide an information recording medium that can attain a more improved resolution.